1. Field of the Invention
The present invention relates to an apparatus for generating power by use of a gas fuel such as gasified coal.
2. Discussion of the Related Art
Coal is of importance as a fuel for power generation now and in the future since there are a lot of coal reserves, and the coal reserves are hardly unevenly distributed over the world. It is required to reduce the emission of the materials such as SOx, NOx, or CO.sub.2 from a power plant which generates power by using coal as a fuel, which adversely affect the global environment, and also to improve the efficiency of power generation. To satisfy such requirements, a coal gasification power generator, a power generator of the pressurized fluidized bed combustion boiler type and the like have been developed in lieu of a conventional pulverized coal boiler. As one of those developing power generators, there has been proposed a power generator using a fluidized bed coal gasifying furnace. One example of the conventional power generator is shown in FIG. 6. This conventional power generator has been proposed by British Coal Corporation in Great Britain, and will be described with reference to FIG. 6.
coal 100, limestone 400, air 203 and water vapor 300 are supplied to a coal gasifying furnace 1'. The coal 100 is gasified in the coal gasifying furnace 1'. H.sub.2 S and COS in the resultant generated gas are reacted with the limestone 400 and fixed as CaS in the limestone 400.
A coal gas 500 generated from the coal gasifying furnace 1' is dust-removed by a dust removing unit 3. A coal gas 501 which has been subjected to dust removal is introduced in a combuster 5. Char which has not been gasified in the coal gasifying furnace 1' and particles 60b of the limestone 400 after the above reaction are extracted from the coal gasifying furnace 1' and then transferred to a hopper 17. Particles 60c retrieved from a coal gas 550 by means of a gas filter 3 which constitutes a dust removing unit is transferred to the hopper 17.
An air 200 is pressurized by an air compressor 6 to produce a pressurized air 201. A pressurized air 204, which is a part of the pressurized air 201, is supplied to the combuster 5. The gas 501 is burned by the combuster 5 by application of the pressurized air 204, thereby generating a gas turbine inlet gas 800. A gas turbine 7 is driven by the gas turbine inlet gas 800 to produce a gas turbine outlet gas 801 having normal pressure. The gas turbine 7 drives the air compressor 6 and the power generator whereby electricity is obtained from the power generator. The heat of the gas turbine outlet gas 801 is retrieved by an exhaust heat recovery boiler 8a to be radiated to the air from a funnel.
The char and particles 60d of a desulfurization agent stored in the hopper 17 are supplied to an atmospheric circulating fluidized bed combustion unit 2' In the atmospheric circulating fluidized bed combustion unit 2', the char is burned by the aid of an air 206 supplied from a blower 18 while CaS contained in the limestone 400 is oxidized into CaSO.sub.4. After the heat of a combustion gas 700 produced from the atmospheric circulating fluidized bed combustion unit 2' is retrieved by an exhaust heat recovery boiler 8b, particles 901 contained in a combustion gas 701 are removed by a dust removing unit 19 so that the gas 700 is radiated to the air from the funnel 9 as a gas 803.
Water vapor 30a which has been heated by heat exchangers 10a and 10b installed in the exhaust heat recovery boilers 8a and 8b and a heat exchanger 10c installed in the atmospheric circulating fluidized bed combustion unit 2' drives a steam turbine 11 which drives the power generator. As a result, electricity is produced from the power generator.
The above-mentioned conventional power generator has a first problem that the efficiency of power generation is low. Among chemical energy possessed by coal, chemical energy possessed by a char 60d which has been transferred to the atmospheric circulating fluidized bed combustion unit 2' without being gasfied in the coal gasifying furnace 1' is transformed into electrical energy by the steam turbine 11. However, the energy possessed by the char 60d is not used for driving the gas turbine 7. Therefore, there is disadvantageous in that the conversion efficiency of chemical energy into electrical energy is lowered for the condition where the gas turbine 7 is not used.
The conventional power generator has a second problem that the efficiency of desulfurization is lowered. The first reason why the efficiency of desulfurization is lowered is that the efficiency of desulfurization of the coal gasifying furnace 1' is lowered. The second reason that the efficiency of desulfurization is that a large amount of SO.sub.2 is emitted from the atmospheric circulating fluidized bed combustion unit 2'.
As described above, in the conventional power generator, limestone and coal are supplied to the coal gasifying furnace 1' so that gasification of coal and desulfurization of H.sub.2 S and COS contained in the gas due to limestone are made in the identical fluidized bed. In this case, it has been recognized that the rate of desulfurization in the coal gasifying furnace 1' is lowered for three causes stated below.
The first cause is that, in an area where oxygen exists at the bottom of the coal gasifying furnace 1', limestone is reacted with H.sub.2 S and COS contained in the gas to produce CaS which is reacted with oxygen. As a result, there occurs a reaction that CaS is analyzed into CaO and SO.sub.2.
The second cause is that a time required for completing desulfurization reaction due to limestone is different from that of gasification reaction of coal. In order to complete desulfurization reaction due to limestone, for example, approximately 120 seconds are required in a gas at 900.degree. C. under the pressure of 12 ata. On the contrary, the time required for gasification of coal is approximately 30 minutes. Therefore, provided that the time necessary for gasification of coal is the particle residence time of coal and limestone in the coal gasifying furnace 1', there is no time sufficient for completion of desulfurization reaction.
The third cause is that, because H.sub.2 S and CO are produced in the whole furnace due to gasification of coal, H.sub.2 S and COS produced in the upper portion of the furnace have a shorter time to be in contact with limestone in the furnace shortened compared with H2S and COS generated at the lower portion of the furnace.
For the above-mentioned causes, the desulfurization efficiency of coal and limestone in the coal gasifying furnace 1' could not be elevated.